"This is something very new," said study researcher Dr. Christine
Manser, a gastroenterologist at University Hospital in Zurich,
Switzerland.

"There has never before been a study investigating the
impact of climate change — represented by an increase in heat
waves — on IBD and IG flares," Manser said.

The researchers defined a heat wave as any period of six or more
days with high temperatures rising above the average daily high
by more than 9 degrees Fahrenheit (5 degrees Celsius).

The study showed that prolonged
periods of hot weather led to a 4.6 percent increase in risk
of people needing to be hospitalized with a relapse of
inflammatory bowel disease for every additional day that a heat
wave lasted.

Manser explained that if extreme heat began on a Monday, and
continued all week, by Saturday (day six) it would be classified
as a heat wave, and by Sunday (day seven) the risk for IBD flares
would increase by 4.6 percent a day.

Infectious gastroenteritis may result in vomiting and stomach
cramps, and can be caused by a virus, such as norovirus; a
bacteria, such as salmonella; or a parasite, such as giardia.

The study is published online today (Aug. 13) in the American
Journal of Gastroenterology.

Heat lag

To find out whether digestive symptoms were linked with heat
waves, researchers looked at the University Hospital of Zurich's
admission records over a five-year period, which included 17 heat
waves.

A total of 738 people with inflammatory bowel disease, and 786
people with infectious gastroenteritis were admitted to the
hospital during these hot spells. Researchers also looked at a
control group of 506 people hospitalized for noninfectious GI
problems, but found no evidence of a heat wave
effect.

The data revealed that when heat waves occurred, they had an
immediate impact on the risk for IBD flare-ups. But the highest
risk for developing infectious gastroenteritis occurred on the
seventh day of a heat wave.

One possible reason for the one-week delay in IG flares is that
heat waves change the bacterial composition of the
gastrointestinal tract, Manser said. But this change in gut
bacteria takes time, which may explain the seven-day time lag in
developing intestinal symptoms, she said.

Manser said several potential mechanisms may explain why IBD
flares in hot weather. One possibility is "that heat waves induce
physical stress, which has been shown to cause flares of
inflammatory bowel disease," Manswer said.

Heat as a tipping point

"I think the study presents an interesting observation, but a
heat wave's overall impact in terms of hospital admissions is a
relatively small one," said Dr. Alan Moss, a gastroenterologist
at Beth Israel Deaconess Medical Center in Boston, who was not
involved in the research.

A 4- to 5-percent increase in hospitalizations may amount to one
or two IBD or IG patients a year, Moss explained.

He noted that air conditioning is more prevalent in the United
States than it is in some places in Europe, perhaps exposing the
Swiss patients to higher outdoor temperatures.

During a heat wave, there's probably a physiological stressor, or
some aspect of diet that changes for IBD patients, that may also
be contributing to flare-ups, Moss said.

"The heat wave may well be a tipping point for IBD and infectious
gastroenteritis," Moss said.

In other words, people with an inflammatory bowel disease, for
example, may experience more bouts of diarrhea as the mercury
outdoor rises. And because hot weather also makes someone likely
to sweat more, that individual could become dehydrated.

"If you're not keeping up with replacing these fluid losses, it's
the dehydration that's making you feel lousier," Moss explained,
prompting some GI patients to head to the emergency room during a
heat wave.